Timber Drying Kilns are used for reducing the moisture content of green sawn timber to a desired level. For best results the timber should be exposed to a sequence of closely controlled atmospheric conditions. This has been achieved in single chamber kilns by holding the timber stationary in the kiln and varying the conditions, or by gradually moving the timber through a chamber in which conditions vary continuously between the inlet and the exit.
Such Kilns typically consist of a single chamber of relatively large capacity and volume. Generally, the larger the chamber the less uniform and precise the conditions experienced by the timber within the kiln. Such kilns require large plant to perform the required heating and cooling operations required within each cycle. As it is difficult to rapidly change and control the conditions within such kilns the emission of volatile organic chemicals may also be a problem. Wood fibres do not increase in temperature substantially until most of the free moisture in the timber is removed. Once the moisture content of timber drops below the “fibre saturation point” (usually about 25% moisture content) volatile organic chemicals may be released when the wood fibres become heated to temperatures above about 120° C. In a standard batch kiln, particularly during high temperature operation, it is difficult to avoid heating the timber above about 120° C. when the moisture content drops below this level.
U.S. Pat. No. 3,902,253, EP122902 and WO85/02250 disclose “drive through chamber” type kilns in which a timber stack is moved between kiln chambers during a drying process. Light weight partitions or curtains are used to isolate adjacent chambers. Due to relatively low thermal insulation provided between chambers the degree to which different conditions may be maintained in adjacent chambers is limited. Further, there is no controlled transfer of the heating medium between chambers. Such kilns may only be suitable for low or medium temperature operation.
U.S. Pat. No. 3,902,253 discloses a five stage kiln suitable for medium temperature operation. The drying time in each stage is four hours making a total drying time of 20 hours. In WO85/02250 FIG. 3 indicates that conditions vary relatively continuously throughout the kiln. WO85/02250 and EP122902 give no specific details as to the operating conditions but appear to be suitable only for low or medium temperature operation.
It is an object of a preferred embodiment of the invention to provide a timber drying kiln which will overcome or at least ameliorate problems in known timber drying kilns, or methods of operating them, or at least to provide the public with a useful choice.